1. Field of the Invention
The present invention relates to a soluble polyimide for a photosensitive polyimide precursor and a photosensitive polyimide precursor composition comprising the soluble polyimide. More particularly, the present invention relates to a soluble polyimide for a photosensitive polyimide precursor that contains hydroxyl groups and acetal groups in its side chains and at least one reactive end-cap group at one or both ends of the polymer chain. The invention also relates to a photosensitive polyimide precursor composition that comprises the soluble polyimide, a polyamic acid containing at least one reactive end-cap group at one or both ends of the polymer chain, a photo acid generator (PAG) and, optionally, a dissolution inhibitor.
2. Description of the Related Art
Recently, high purity organic materials with high processing ability have been studied as materials useful for electronic devices since electronic devices, such as semiconductors and liquid crystal display (LCD) devices, have been required to have higher integrity or density, and to be more reliable and more speedy. Suitable materials for electronic devices must exhibit thermal stability even under manufacturing conditions above 200° C., excellent mechanical strength, low dielectric constant, high insulation characteristic, good planarizing properties, and also have a low content of impurities that can cause damage to the reliability of the devices. If necessary, the materials should be capable of being easily processed into a fine shape. Since polyimides meet the above criteria, special attention has been paid to the polyimides.
Generally, a polyimide is synthesized through a two-step polycondensation: (i) a first polymerization is carried out by polymerizing diamine and dianhydride in a polar organic solvent such as NMP, DMAc and DMF to obtain a polyimide precursor solution; and (ii) a second imidization is carried out by heating the polyimide precursor solution or film to obtain a polyimide in its cured form. The polyimide thus prepared is a type of engineering plastic, and has many advantages such as good mechanical properties, high heat resistance, high insulation and good planarization properties.
FIG. 1 illustrates an example of the use of a polyimide resin for an electronic device. As shown in FIG. 1, a resin molded LSI includes a polyimide buffer coating layer of 10 μm or more (in thickness), which is inserted between a chip and a packaging material in order to prevent a passivation layer of the chip from being cracked, or to prevent a metal wiring from being damaged by volume shrinkage of the resin, and by a possible difference between heat expansion coefficients of the chip and the resin after molding. The polyimide buffer coating layer should have fine patterns for an inter-electrode connector and a wire bonding on its surface. For forming the fine patterns on the surface, in the conventional method, a photoresist is coated onto a polyimide film and is removed after the etching process. However, since the etching process incurs considerable cost and is liable to cause many defects, earnest studies have been made on imparting photosensitivity to a polyimide itself.
For example, U.S. Pat. No. 3,957,512 issued to Rubner, et al. from Siemens AG discloses polyamic acids to which a photosensitive functional group is bound through ester bonding; and U.S. Pat. No. 4,243,743 to Toray industries, Japan, discloses a polyamic acid as a precursor for a photosensitive polyimide, which has a photosensitive functional group and an ionically-bound amino group. These two patents disclose a negative-type photoresist composition, and so the composition containing the polyimide or its precursor is coated on a substrate and partially exposed to light under a photo-mask in order to obtain a desired pattern. By irradiation, the exposed parts become insoluble owing to photopolymerization and photo-cross-linking between the precursors and, through a developing process with organic solvent, a desired pattern is made and then goes through further imidization reaction to obtain a polyimide layer having a prescribed pattern.
However, when compared with a positive-type photoresist, not only does the above negative-type photoresist have lower resolution and greater risk of defects, but because it requires the use of an organic solvent (i.e. NMP or DMAc) as a developer, it is not preferred from an economical or environmental point of view. Therefore, there has been extensive research regarding polyimides to be used as a positive-type photoresist composition.
For example, Japanese Patent Laid-Open Nos. 52-13315 and 62-135824 disclose a method of forming a pattern from a mixture of a polyamic acid (as a polyimide precursor) and naphthoquinonediazide (as a dissolution inhibitor) by using the difference between dissolution rates of an exposed portion and a non-exposed portion. Japanese Patent Laid-Open No. 64-60630 discloses a method of using a mixture of an organic solvent-soluble polyimide resin having hydroxyl groups and a naphthoquinonediazide compound. Japanese Patent Laid-Open No. 60-37550 discloses a photosensitive polyimide, which has a photosensitive group of o-nitrobenzyl group onto a polyimide precursor through an ester bonding. Japanese Patent Laid-Open Nos. 7-33874 and 7-134414 disclose a chemically-amplified composition comprising a resin and a Photo-Acid Generator, the resin being obtained by converting carboxylic groups of a polyamic acid into functional groups capable of being dissociated by an acid.
The photoresist compositions described in the above documents have many drawbacks. First, these conventional photoresist compositions require a large amount of photosensitizer since the difference in dissolution rates between an exposed portion and a non-exposed portion is not large enough to form a pattern with high resolution. In addition, when coated on a substrate, the planarization characteristics of these conventional photoresist compositions are not satisfactory. Moreover, despite adding the large amount of the photosensitizer, the sensitivity is still inadequate in many cases, and thus a thick polyimide layer cannot be easily attained. Further, even in case where the thickness of resulting film is acceptable, severe film shrinkage occurs during curing and many of the physical properties of the obtained polyimide film are not good, so the material is not thought to be suitable as a buffer layer. For those reasons, conventional photosensitive polyimides have still been limited in their practical applications until now.
The description herein of disadvantages associated with known materials, methods, and apparatus is in no way intended to limit the present invention. Indeed, various features of embodiments of the invention may employ one or more known materials, methods, and apparatus without suffering from the above described disadvantages.